This invention relates to dispensing apparatus for use in dispensing liquid as an atomised spray and in particular but not exclusively to medical atomisers.
It is known to produce a stream of liquid droplets by vibrating a perforate membrane having a rear face contacted by liquid such that droplets are expelled from holes in the membrane at each cycle of vibration. The size of droplet produced will depend upon the hole size and for practical purposes the thickness of the membrane will tend to be of the same order of magnitude as the hole size. Consequently it has hitherto not been practicable to use such apparatus for the production of an atomised spray for use in applications such as medical inhalers where for example droplet size of less than 10 microns may be required.
It is known from U.S. Pat. No. 4,533,082 to provide dispensing apparatus comprising a housing defining a chamber receiving in use a quantity of liquid to be dispensed, the housing comprising a perforate member which defines a front wall of the chamber and which has a rear face contacted by liquid in use, the apparatus further comprising vibrating means connected to the housing and operable to vibrate the perforate membrane to dispense droplets of liquid through the perforate membrane.
The co-pending application discloses a new and improved dispensing apparatus in which the housing comprises an annular member having a relatively thin inner annular portion connected to the perforate membrane and a relatively thick outer annular portion connected to the vibrating means.
An advantage of this arrangement is that the vibrating means is presented with a relatively high acoustic impedance compared with a relatively low impedance found in the inner annular portion so that the amplitude of vibration transmitted to the perforate member is amplified during transmission of transverse acoustic waves through the annular member.
An efficient arrangement is thereby attainable when vibrating the perforate membrane at the higher frequencies favoured for the production of smaller droplets.
Preferably the annular member comprises a disc defining a central aperture bounded by the inner annular portion and traversed by the perforate membrane and wherein the thickness of the disc tapers in a radially inward direction.
Such a disc may be regarded as an impedance transformer in which the outer annular portion is matched in impedance to a vibrating means such as a piezoelectric transducer and the inner annular portion is matched in impedance to the perforate membrane.
Advantageously the disc has front and rear faces which converge at an angle of taper which varies with radius such that the inner annular portion has a smaller angle of taper than that of the outer annular portion.